1. Field of Invention
The present invention generally relates to a heat exchange apparatus, and more particularly to a direct expansion evaporator for a refrigeration system, wherein the direct expansion evaporator provides two heat-exchange stages for making a frozen product from raw material while being energy efficient.
2. Description of Related Arts
A direct expansion evaporator for heat exchanging, such as a heat exchanger, is commonly and widely applied in lots of areas, such as industrial or medical application. For example, the heat exchanger is used in a chemical factory for cooling down a waste to prevent overheating the waste. Heat exchange is also applied in refrigeration, freezer system, or air conditioner for exchanging the heat from the air or the frozen product to keep it under certain temperature.
Most of the heat exchange used for cooling system applies a refrigerant to provide a lower temperature source, which has the temperature below to the targeted object for being cooled, so that the heat exchanging is driven by the driving temperature between the refrigerant and the object to achieve the cooling purpose.
Traditionally, the refrigerant is stored in a high pressure container and released via an expansion valve to a heat exchange chamber, which has lower, pressure than the high pressure container, so that the refrigerant is rapidly evaporated to gaseous phase due to the pressure drop, so as to provide the thermal energy to the heat exchange chamber. The phase change from liquid to gas of the refrigerant absorbs dramatic heat energy, so that the targeted object is able to be cooled in a short period. Therefore, the heat exchanger plays an important role of using the refrigerant for a cooling system, such as for making a frozen product.
In other words, evaporator is a critical heat-exchanging component in the refrigeration system. It is a decisive factor of the system capacity and efficiency. When liquid refrigerant enters the evaporator through the expansion valve or a capillary tube, it rapidly vaporizes due to the sudden expansion of volume and reduction of pressure. During this vaporization process, the refrigerant absorbs heat from the cooling medium through the evaporator wall or housing with good thermal conductivity.
There are several types of evaporator commonly used for cooling system in the existing market: coil type, fin type, and spiral channel type. The coil type evaporator has a copper tube filling with refrigerant and a feeding tube for containing raw material therein. The copper tube is adapted at a position that the copper tube is winding at the outer surface of the feeding tube, so that the thermal energy is transferred through the walls of copper tube and feeding tube for heat exchanging. Therefore, the heat exchange takes place through not only the wall of the feeding tube but also the wall of the copper tube.
However, the contacting area between the copper tube and the feeding tube is limited. The heat is conductively transferred through two layers, the walls of copper tube and feeding tube, so that the heat transferring is inefficient.
The fin type heat exchanger has a plurality of fins welded on the refrigeration cylinder to provide alternatively channels for the flow of the refrigerant from the inlet to the outlet so as to prolong the traveling distance and time of the refrigerant. Although it has higher heat transfer rate to perform a relatively more efficient heat exchanging, it is complicated in fabrication procedure. Thus, it also results in high manufacturing cost and low productivity.
In order to increase the efficiency of heat exchanger, the spiral type heat exchanger is also provided for enhancing the heat transferring rate. The spiral type has threads on the inner surface of outer cylinder enclosing the feeding pipe therein, so that the refrigerant is flowing in the spiral threads channel between the feeding pipe and the outer cylinder. Though it enhances the heat exchanging efficiency, the spiral threads outer cylinder is relatively longer and needs precise assembly process. Thus, it is high in manufacturing cost and not suitable for batch process or mass production.
Therefore, using the direct expansion evaporator or heat exchanger to exchange heat for making a frozen product needs a relatively more efficient expansion evaporator or heat exchanger. Take ice cream as the frozen product for example. The ice cream raw material made in the traditional spiral type can not efficiently provide the thermal energy that needed for freezing the ice cream to a desired hardness. The spiral type does not have enough expansion area in the heat exchange channel for release the thermal energy in the expanded evaporation manner. Thus, the spiral type of direct expansion evaporator uses relatively more power and is not efficient for using in batch process for producing a massive amount of frozen product.
For another example, frozen yogurt needs more thermal energy to form the final frozen product, otherwise, it usually tempts to be too soggy or liquefied to become the frozen yogurt. The existing heat exchangers of direct expansion evaporator cannot provide an efficient and capable of producing a relatively larger amount of frozen products. Therefore, it is an attempt of the present invention to provide a more efficient direction expansion evaporator, so as to prevent the energy waste and make better quality of frozen product.